Maintaining Continuity – The Axis

If you have action in a scene, continuity means the motion must flow in the same direction with each successive shot unless you show a change in that direction. For example, shots of a car moving left-to-right should maintain the same left-to-right direction between wide shots and close-ups. In the close-up shot, the driver must be facing the same left-to-right direction as the car in the wide shot. If the driver turns the car in another direction, the camera must show the car turning. Only then can you cut to the action moving in a different direction without causing confusion in the viewers.

The Axis (Vector Line)

The Axis is an imaginary line that extends in the direction of the subject. When the camera is on one side of the axis, the subject's direction is a certain way. But when the camera crosses the axis, the subject's direction, relative to the camera, changes. If the two shots are edited back-to-back, the change in direction becomes even more obvious. 

Think of a chase scene where one person chases another; each shot shows them moving in the same direction. But if the camera crosses the axis for one person, then you no longer have a chase, but instead you have a game of chicken. 

The axis is prominent especially when shooting sports. For example, in a football match where the player runs in a specific direction to make a touchdown, then it's important to maintain their direction so the viewer knows they're heading for the proper goal. If there are cameras on opposite sides of the axis and the editor or director cuts from both cameras, one shot will show the player running to the right and the other shot to the left. Which direction is the goal? The result of this cut adds confusion in the minds of viewers... and they will lose interest in the show as a result. 

The 180 Degree Rule

The camera is kept on one side of the line and can sweep 180 degrees to get other shots without changing the subject's direction. The framing of shots conforms to what is called the 180 Degree Rule 

Wikimedia Commons 

Crossing the Axis

In an edited sequence, when the camera jumps across the axis, the subject in one shot might appear to change direction in the next. Crossing the axis breaks the continuity that was established by the first shot, which can be confusing to your audience. Violating this continuity is also known as "crossing the line" or "crossing the vector."

The video below shows what happens when the camera crosses the axis. Published by Richard Schaefer, University of New Mexico.

This video offers a good explanation of the 180 Degree Rule, published by Marcelo Paulo De Souza of YouTube's Film Project.